Why Do Stars Twinkle While Planets Do Not?

Stars, except for the Sun, although they may be millions of miles in diameter, are very
far away. They appear as point sources even when viewed by telescopes. The
planets in our solar system, much smaller than stars, are closer and can be resolved as
disks with a little bit of magnification (field binoculars, for example).

Since the Earth's atmosphere is turbulent, all images viewed up through it tend to
"swim." The result of this is that sometimes a single point in object space gets
mapped to two or more points in image space, and also sometimes a single point in object
space does not get mapped into any point in image space. When a star's single point
in object space fails to map to at least one point in image space, the star seems to
disappear temporarily. This does not mean the star's light is lost for that
moment. It just means that it didn't get to your eye, it went somewhere else.

Since planets represent several points in object space, it is highly likely that one or
more points in the planet's object space get mapped to a points in image space, and the
planet's image never winks out. Each individual ray is twinkling away as badly as
any star, but when all of those individual rays are viewed together, the next effect is
averaged out to something considerably steadier.

The result is that stars tend to twinkle, and planets do not. Other extended
objects in space, even very far ones like nebulae, do not twinkle if they are sufficiently
large that they have non-zero apparent diameter when viewed from the Earth.